1. Field of the Invention
The present invention relates generally to the field of circulating fluidized bed boilers and, in particular, to a new and useful configuration for reducing or eliminating tube erosion in the region of the top of the refractory covering on lower furnace walls, or on wing walls or division walls.
2. Background of the Invention
In circulating fluidized bed boilers, the problem of erosion of tubes at the top edge of refractory lining is well known.
In a circulating fluidized bed boiler, reacting and non-reacting solids are entrained within the enclosure by the upward flow of gases which carry some solids to the reactor exit at the upper end of the reactor. Other, larger quantities of solids are recycled within the reactor enclosure as heavier solids initially carried upwards fall back against the flow of gases. Since the velocity of the upward flow of gases is often much lower in the cooler gases adjacent the circulating fluidized bed enclosure walls and heat transfer surfaces within the circulating fluidized bed, most of the solids fall near the walls or heat transfer surfaces.
The amount of solids falling adjacent to the walls and surfaces increases progressively toward the bottom of the circulating fluidized bed. The density of the bed is higher in the lower regions of the furnace, and as a result, the walls and surfaces in the lower regions are subject to increased erosion from contact with the solids.
Further, the reactions occurring in the circulating fluidized bed create chemical reduction conditions against which the walls and heat transfer surfaces must be protected. A protective material (further called refractory) is often used to coat the walls and exposed surfaces in the lower regions of the circulating fluidized bed. The refractory material, anchoring and installation is expensive, since it must withstand high temperatures (typically between 1400xc2x0 and 1800xc2x0 F.), contact erosion from solids, and chemical reduction and by-products from the combustor reactions. The refractory also reduces the efficiency of the heat transfer. For this reason, refractory is only applied to the walls and exposed surfaces to as low an elevation in the reactor region as possible considering corrosion and erosion conditions. At the point on the walls and surfaces where the refractory terminates, a discontinuity is formed where erosion of the metal of the tubes forming the walls occurs. The erosion is typically in a band about xc2xc to 3xe2x80x3 wide adjacent the top edge of the protective material. Tube wall erosion is found in an area between 0 and 36 inches above the top of the refractory.
One method for reducing this erosion is found in U.S. Pat. No. 5,893,340 to Belin et al. in which the walls of the enclosure are bent into and out of the solid flow stream to reduce the incidence of solids on the refractory discontinuity.
An alternative known method is to place a protective overlay material on the tube at the refractory discontinuity as a shield. The protective overlay extends from below the termination of the refractory to several inches above the discontinuity. Unfortunately, the protective overlay suffers the same erosion and must eventually be replaced in an expensive and time consuming procedure.
None of the prior methods are completely successful in eliminating erosion near the refractory.
It is an object of the present invention to provide an efficient alternative tube section design for a wall, wing wall, or division wall which reduces tube erosion adjacent a refractory discontinuity in a circulating fluidized bed boiler.
Accordingly, one aspect of the present invention is drawn to a tube wall section for a circulating fluidized bed boiler which has a swaged section of tubes above a refractory discontinuity partly covered by an abrasion-resistant refractory tile or shaped refractory. The refractory tile or shaped refractory is mounted over the swaged section and a lower adjacent reduced diameter tube section covered by the refractory. The membrane bar between adjacent tubes is modified in the swaged tube section and reduced tube diameter section to permit mounting of the refractory tile or shaped refractory over the tubes. A mirror image swaged section may be provided below the reduced diameter tube section to bring the tube back to the original or another diameter in the tube wall covered by refractory.
The refractory tile may be mounted in one of several alternative ways. In one embodiment, bolts or studs, and nuts, may be used to secure the refractory tile. Alternatively, locking clips which are connected to the bottom of the refractory tile segment may be used. A locking tab mount may be used with the locking clips. The tabs extend upwardly between adjacent swaged tube sections where the tabs are held between the modified membrane bar and the regular membrane bar to secure the refractory tile in place. The shaped refractory is held in place by studs and anchors welded to the tubes and membrane.
The original tube diameter above the tapered portion of the swage and the inner surface of the membrane bar define the fall line for solids within the circulating fluidized bed, while the swaged tube section with the modified or displaced membrane bar creates a space which is outside the fall line. The protective abrasion resistant refractory tile or shaped refractory resumes the fall line and covers the exposed tube sections down to the refractory. The top edge of the refractory tile or shaped refractory is outside the fall line as well, so that the discontinuity line is not simply moved upwards.
In another aspect of the present invention, the above-described concept is applied to refractory discontinuities on wing walls or division walls located within the furnace of a circulating fluidized bed boiler. As will be described later, in such applications the refractory tiles would be shaped slightly differently and applied back to back on both sides of the section comprising the wing walls or division walls. Where the membrane bar is stepped back for the enclosure walls, it is simply stopped, leaving a gap, for such wing walls or division walls inside the furnace.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.